The production of olefins from C.sub.3 -C.sub.5 hydrocarbons is well known. The technology for producing olefins is available commercially from several sources, such as, for example, the Catofin/Catadiene process, the UOP Oleflex process, and the like. The processes typically involve selective catalytic dehydrogenation of a saturated hydrocarbon stream at elevated temperature. The primary differences between the commercially available technologies are generally in the configuration and operation of the dehydrogenation reactor, but each generally requires heating the hydrocarbon feed to the reactor and cooling the olefin-rich effluent from the reactor. Various approaches to recovering heat from the reactor effluent have included the production of steam and heat exchange with the feed. Even with these attempts at heat recovery, it has been necessary to provide additional heat for the hydrocarbon feed stream and cooling water for further cooling of the effluent stream. The shortcomings of the prior art heat recovery efforts are manifested in excessive cooling water, fuel and steam import requirements.
A typical prior art catalytic dehydrogenation unit design is schematically illustrated in FIG. 1. A feed stream 10 comprising mainly C.sub.3 -C.sub.5 saturated hydrocarbons at ambient temperature is heated in a low pressure steam heat exchanger 12 to produce a vaporized feed stream 14. The vaporized feed stream 14 is heated against hot reactor effluent in exchanger 16 to produce a partially heated feed stream 18 which is heated to reaction temperature in a fired preheating furnace 20 to produce a hot feed stream 22 for the reactor 24.
The reactor 24 is typically an adiabatic, fixed-bed catalytic dehydrogenation reactor in which the saturated hydrocarbons in the feed are dehydrogenated to produce an olefin-rich stream 26. Catalyst regeneration is effected cyclically by using parallel reactors which are alternately taken off stream for regeneration, or continuously using reactors operated in a stacked series with continuous catalyst withdrawal for regeneration and recycle. Regardless, conversion to the desired olefin product is primarily controlled by reactor pressure and temperature. Because olefin yield is enhanced by lower pressures, the reactor 24 is typically operated at approximately atmospheric or subatmospheric pressure.
The hot effluent stream 26 is typically used to generate steam in exchanger 28. Furnace 20 flue gases, along with reactor regeneration gases can also be used for waste heat recovery for steam production. A partially cooled product stream 30 is supplied as the hot-side fluid to process heat exchanger 16 and further cooled in cooler 32 to a suitable temperature for compression. The cooler 32 can be an air cooler, but usually uses water, typically supplied from a recirculating cooling tower, as the heat exchange medium.
Because it is necessary to avoid excessive temperatures in the compression of the product stream to enhance compressor efficiency, compression is typically effected with a plurality of staged compressors 34, 36, 38 using intercoolers 40, 42 in series so that each compressor discharge has a temperature below about 260.degree. F. and each compressor suction has a temperature of about 90.degree.-125.degree. F., although care is usually taken to ensure that the product is maintained in a gaseous phase. If desired, any condensate formed, for example, in the intercooler 42, can be removed using a separator (not shown) which is conventional for this purpose. An aftercooler 44, cools the high pressure product stream 46 to the desired temperature for subsequent finishing in section 48, typically including absorption, stripping and stabilization, to produce fuel gas stream 50 and an olefin-rich product stream 52.